Manganese is widely used in the iron and steel industry, batteries and other fields. The amount of the manganese oxide ores existing in China coeval with iron and their manganese-to-iron ratio is low. A study was conducted on a synchronous reduction roasting-magnetic separation of a ferromanganese ore with high content of iron taken from South Africa using CO as reductant. Under the conditions of 750°C roasting temperature, 40 min roasting time, 30% CO concentration roasting atmosphere and − 0.074 mm accounting for 82.56% grinding fineness, the iron removal rate was close to 70% through magnetic separation, with the grade and manganese recovery of manganese concentrate reaching 49% and 72.50% and the manganese-to-iron ratio increasing from 2.5 to 5.92. The X-ray diffraction analysis indicated that Mn2O3 and Fe2O3 in the raw ore was reduced to MnO and Fe3O4, respectively, without forming the intermediate product Mn3O4 in a weak reducing atmosphere. The SEM-EDS analysis of the roasted product showed that the simultaneous reduction of manganese and iron oxides could be achieved when the roasting temperature reaches 750°C. The crystal forms of MnO and Fe3O4 were more perfect, and the coexisting grains of Ca, Si and Al clustered further with increasing roasting temperature.